The present invention relates to a method and apparatus for fabricating a read only optical disc such as compact discs and video discs, and more particularly to a method and apparatus for fabricating a read only optical disc for performing optical duplication by utilizing a source disc wherein write holes for transmitting light are arranged depending on information and a target disc having a photochromic write layer, to thereby provide the duplicated target disc as a read only optical disc.
Besides compact discs and video discs currently in wide use, CD-ROMs having character information, and compact video discs wherein video and audio signals are written, being distributed on an area of a regular ratio can be given as examples of read only optical discs. These discs generally has a mirror for reflecting light and countless write pits formed in the mirror surface. Upon projecting light to such a disc, the mirror surface without write pits brightly reflects the light, while the write pits reflect a slightly dimmed light beam. The radiation intensity in the reflected light is thus varied by scanning, so that the information can be read out by detecting this variance.
All fabricating processes of the above-described read only discs are basically similar. Here, a fabricating process of a compact disc is shown in FIG. 1, which includes a master fabrication step 100, a stamper fabrication step 200, a disc substrate molding step 300, and post-processing step 400.
To begin with, in master fabrication, a photoresist 2 is coated on a glass disc 1 with a well-ground surface, and a master 4 formed with write pits 3 is fabricated by cutting photoresist 2 via exposure and development according to the information to be written. Here, an apparatus called as a "mastering machine" (refer to U.S. Pat. No. 4,953,152) is utilized dung the cutting process, wherein a modulated laser beam is projected which carries the desired information, thereby exposing the corresponding portions of write pits 3.
In stamper fabrication, a metal layer 5 is thinly deposited on master 4, and a nickel layer 6 and a metal substrate 7 are sequentially formed on metal layer 5. Then, master 4 is separated to thereby complete a stamper 8.
In disc substrate molding, an ultraviolet-curing resin 10 is injected between stamper 8 and a plate member, 9 and pressed. Then, an ultraviolet ray is projected. Thereafter, when stamper 8 and plate member 9 are separated from each other, a hardened disc substrate 11 having write pits 3' (which are the same as master 4) is obtained.
Next, an aluminum reflective layer 12, a protection layer 13 and a label 14 are sequentially formed on disc substrate 11. Then, by carrying out post-processing, e.g., the formation of a center hole 15, a compact disc 16 is completed.
Any of the above-described processes must be strictly managed in a clean room. Especially in the master fabrication, since the thickness of the photoresist is 0.12 .mu.m which must be processed within an error range of .+-.5 nm, ambient concerns (i.e., dust, small variations in temperature and humidity, vibration, etc.) are more strict than those maintained curing semiconductor manufacturing processes. In cutting, since the width (e.g., 0.4 .mu.m) and interval (e.g., 1.6 .mu.m) of a track in which write pits are formed are very minute, a costly and highly accurate mastering machine having advanced control-performance must be employed so as to precisely project the laser beam. In disc substrate molding, a highly developed molding technique is required for forming submicron write pits in the disc substrate. In the meantime, the number of disc substrates moldable by a single stamper is limited due to the abrasion of the stamper.
In other words, the conventional method for fabricating a read only optical disc requires several process steps and the use of an expensive apparatus and advanced technique, which is not economical. Accordingly, a read only optical disc cannot be supplied at a low price. Also, the read only optical disc is difficult to be practically fabricated on a short-run production basis.